Oral self-administration of ethanol and cocaine in rats

Most laboratory animal studies on self-administration of drugs of abuse use only one drug, whereas humans frequently engage in polydrug use. For this reason, we studied oral self-administration of ethanol (E) and cocaine (C) with the free choice bottle method using a single drug alone, a combination (E and C in separate bottles) or a mixture of both drugs in a single bottle. Young female rats (45 days) consumed similar amounts of C if offered alone (12.4 +/- 7.5 mg/kg/day), in the presence of ethanol (10.6 +/- 3.5) or as E/C mixture (8.0 +/- 4.0). They also consumed similar amounts of E if offered alone (3.8 +/- 1.6 ml/kg/day), in the presence of C (2.3 +/- 0.8) or E/C mixture (2.4 +/- 1.1). Voluntary consumption of both drugs varied markedly among animals but was consistent in a given rat. No correlation occurred between consumption of E and C. Young male rats behaved similarly and consumed similar amounts of E and C alone, in combination and as mixture. While E consumption was similar, C consumption was higher in female rats. Old male rats (180 days) were similar to young male rats. The presence of a saccharin solution as a distracter had no effect on intake of E or C in young females but reduced E intake only in young male rats. In young animals, prior voluntary consumption of either E or C had no effect on subsequent voluntary consumption of the same or other drug offered in combination. These results indicate that this model may be useful to study polydrug use in humans, that consumption of both E and C is strongly controlled by an individual animal, that prior exposure to one drug had no or little effect on a subsequent consumption of the same or other drug in combination and that intake of E or C seems to be independent of each other suggesting two independent reward centers.